Comment: A new e mail rumor indicates that 9 volt batteries may cause a
fire if short circuited. This seems plausible, except: I short circuited
2 batteries (fully charged), together, (minus to plus and plus to minus,
simultaneously) and the batteries demonstrated a temperature rise barely
sufficient to detect with the bare finger, let alone cause a fire.

likewise, making a "loose" connection to cause a spark is nearly
impossible.

I suspect this is someone's "worry" rather than an actual fact.

My question is: Is there enough energy in a 9-volt battery to start a
fire? If so, how?

This assertion, (starting a fire) may be theoretically correct, but in
real life seems to be not on the probability scale.

A 9V battery and a hunk of steel wool is sometimes used as a fire starter. The mass of the battery itself is too great to get it to heat enough to cause of fire even with a dead short across the terminals. The steel wool has high enough resistance and low enough heat capacity that it'll go incandescent within a second of two. That battery will barely change temperature.

When I was a kid I was playing around with a 9V battery and decided to short circuit it with a paperclip just to see what would happen. While it probably wasn't hot enough to start a fire, the paperclip definitely got hot enough to be painful.

If you are storing loose 9 volt or AA or other batteries in a kitchen drawer or a “junk” drawer in your home, watch how you store them. Above all don’t store them loose and rolling around with other metal, glues and more of the lovely mix of things we keep in our junk drawers.

All you need to have happen is for a metal object like steel wool or a paper clip short out across the top of a 9 volt battery and ignite paper or other easily ignited materials and you’ll have a potential disaster in your home. As indicated on the YouTube Video below, it doesn’t take much to heat a metallic object or cause a spark in order to start a fire.

A 9V battery can not "spark" (as in an electrical spark) and ignite something. You can't get a significant spark at a voltage that low now matter what the configuration of objects. AA through D batteries are even lower voltage so the chance of a electrical spark capable of igniting something (even a combustible gas) are zero.

The only thing that'll get hot enough to ignite paper (or a combustible gas) would be something like steel wool. A paper clip won't do it, neither will a metal spoon, or anything else that is thick, that might be in a junk drawer.

Putting tape over the terminals when discarding is OK but really more trouble than it is worth. If you are concerned just touch the terminals to a solid piece of metal (like a spoon) for a couple seconds. The spoon will barely get warm (it has too much mass and conducts heat too well to get very hot) and the battery is now "dead". It is dead forever and won't be a fire hazard in the trash (or in the recycling center's bin).

A more important reason for being careful when storing batteries is because they are expensive. A brand new $2 9V battery rolling around in a drawer with metal objects is going to be dead when you want to use it.